Drug use is an important problem in the US. However, the complex and multifaceted mechanisms that lead to and support drug abuse are only vaguely understood. Increased impulsivity is commonly observed in drug users and is thought to be an important trait of individuals who have problems with illicit drugs. Impulsivity can be quantified experimentally using temporal discounting paradigms. Temporal discounting refers to the phenomenon that a reward, which occurs in the present time is valued more than rewards that occur some time in future. A poorly studied aspect of temporal discounting (and therefore impulsivity) is how individuals process time. Therefore, our proposal aims to examine the relationship between temporal discounting and subjective and objective measures of time perception in young adults (age 18-21) who use stimulants and are of risk for transition to stimulant dependence. The proposed study is based on the hypothesis that these individuals, relative to age and education matched comparison subjects show enhanced temporal discounting due to an altered sense of time. The specific aims are: (1) we will evaluate the relationship between delayed discounting in a decision making task and in subjective and objective measures of time perception in young adults who use stimulants and in matched controls; (2) we will evaluate the relationship of functional magnetic resonance imaging activation between delayed discounting in a decision making task and a time estimation task in young adults without stimulants use. These studies are based on the assumption that impulsivity, measured with temporal discounting, is dependent on a subjective focus on the present time and an overestimation of the duration of time intervals. We will use functional magnetic resonance imaging during temporal discounting and time estimation tasks in healthy volunteers to elucidate whether similar neural substrates underlie both processes. It is proposed that examining how individuals process time to how they process current versus future rewards using functional neuroimaging enhances our understanding of the biological underpinnings of impulsivity. Moreover, a better understanding of the neural mechanisms underlying temporal discounting may provide us with a marker to study the biological basis of risk for addiction. The current study will inform treatment programs to create intervention strategies that manipulate the temporal delay of rewards to shape more adaptive behavior. [unreadable] [unreadable] [unreadable]